Background and goals: Hypertensive disorders affect 300,000 pregnancies in the US each year, increasing the risk of many adverse outcomes including maternal and fetal death. Treatment of severe hypertension in pregnancy is routine. Treatment of mild to moderate hypertension with medications is controversial, however, because of uncertainty about the amount of benefit and concern that medications may harm the fetus. Little rigorous evidence exists to help women and their clinicians weigh treatment risks and benefits. Randomized trials have been small in size, limiting their ability to provide conclusive answers, particularly for comparisons of different medications. Observational studies thus far have lacked data on actual blood pressure levels, preventing them from accounting for differences in hypertension severity between women treated in different ways. No studies have investigated whether risks and benefits of treatment differ by maternal race/ethnicity or obesity status. This study will fill these gaps by using real-world clinical data on pregnancies complicated by mild to moderate hypertension to determine how maternal and infant outcomes differ in treated and untreated women as well as between medication classes and subgroups of women. Results will provide important new evidence to support more informed and effective treatment decisions. Methods: This project is a retrospective cohort study set within 3 integrated health care delivery systems (Kaiser Permanente Northern California, Kaiser Permanente Southern California, and Group Health) that maintain comprehensive electronic medical records (EMRs), including extensive data on blood pressure values before and during pregnancy. Health plan data for 33,000 pregnant women with mild to moderate chronic or gestational hypertension delivering from 2007 through 2014, linked to state birth and fetal death certificate data, will be used to address these aims: 1) Evaluate the effect of pharmacologic treatment, compared to no treatment, on important adverse maternal and infant outcomes, while taking into account differences in hypertension severity and other factors between the groups; 2) Compare the risks of clinically important outcomes with different antihypertensive medications; 3) Determine whether the benefits and risks vary by maternal race/ethnicity or obesity status. The primary outcomes to be studied are preeclampsia, preterm birth, small size for gestational age, congenital malformations, maternal or neonatal intensive care unit admission, and fetal or infant death. Impact: This research will determine risks and benefits of treating mild to moderate hypertension in pregnancy. Results may immediately affect clinical practice, helping women and providers make more informed choices together. Ultimately, evidence-based treatment will improve public health by helping clinicians protect maternal health while preserving fetal growth and promoting delivery at or near term.